PROJECT 4: SUMMARY/ ABSTRACT The purpose of this proposal is to provide critical pre-clinical and early phase clinical evidence to determine whether the Spleen Tyrosine Kinase (SYK) is a promising therapeutic target in ovarian cancer. We have previously compared the proteomes of primary and recurrent/post-chemotherapy ovarian high-grade serous carcinoma (HGSC) tissues from the same patients. Among the preferentially expressed proteins in recurrent HGSCs, a non-receptor tyrosine kinase, SYK, was prioritized for study because small molecule inhibitors of SYK including fostamatinib are available for pre-clinical testing and clinical trials. We were able to validate overexpression of SYK and its active (auto)phosphorylated form in recurrent HGSC after carboplatin and paclitaxel treatment compared to treatment naive tumors. SYK inhibition exhibited a synergistic cytotoxic effect with paclitaxel, docetaxel, and vinorelbine, all of which target the microtubule network. Paclitaxel resistant ovarian cancer cells exhibit higher levels of SYK expression than their carboplatin-resistant counterparts. Our preliminary phosphoproteomic analysis revealed tubulins and several microtubule-associated proteins as SYK substrates in ovarian cancer cells. Phosphorylation of these proteins has been shown to increase microtubule dynamics, a process antagonizing the microtubule-stabilizing effect of paclitaxel. In a mouse tumor xenograft model, the combination of R406 (the active form of fostamatinib) and paclitaxel significantly suppressed tumor growth without overt signs of toxicity. Our pre-clinical studies support a novel hypothesis that SYK activity is required for paclitaxel resistance and that SYK inhibition sensitizes HGSC to the cytotoxic effect of paclitaxel. Therefore, SYK inhibitors represent a promising new strategy to treat ovarian cancer. To test the above hypotheses, we propose the following Specific Aims: Aim 1. Phase I/Ib clinical trial of combined Fostamatinib and paclitaxel in ovarian cancer. Aim 2. Characterize the prioritized SYK substrates discovered in ovarian cancer cells. Aim 3. Assess the efficacy of SYK-based combination therapy in mouse models..